The invention relates generally to drive mechanisms and more particularly, to external rotor drive mechanisms used in pumping intravenous infusion fluids.
Positive pressure infusion systems include a pumping mechanism which forces the infusion fluid from a fluid supply through a fluid conduit to a patient. The fluid supply may take various forms including a collapsible bag, a bottle or a syringe. One typical type of infusion pump includes a peristaltic pumping mechanism for creating a moving zone of occlusion in the fluid conduit to move the infusion fluid through the conduit to the patient.
In typical peristaltic pumping mechanisms, a stepper or other type of motor supplies rotational drive motion which is translated to linear drive motion by means of a peristaltic drive mechanism including peristaltic fingers which themselves form the moving zone of occlusion. The output of the stepper motor is typically coupled to a cam shaft by means of a drive belt or gears. Coupled to the cam shaft is a set of cam followers or peristaltic fingers which are mounted to convert the rotational drive motion of the cams into linear drive motion. These peristaltic fingers move against the fluid line to create the moving zone of occlusion and move the fluid through the fluid line.
An example of one such mechanism is shown in FIG. 1. A pumping mechanism 10 includes a stepper motor 12 used to drive a peristaltic mechanism 14 through a drive belt 16. The peristaltic mechanism includes a plurality of peristaltic fingers 18 coupled to a respective plurality of cams 20. The output shaft 22 of the stepper motor 12 turns an attached drive gear 24 which moves the drive belt 16. A drive pulley is attached to the cam shaft 28 of the peristaltic mechanism 14 to rotate the cams 20. Each cam 20 drives a particular peristaltic finger 18. The angles between adjacent cams are selected so that the peristaltic fingers occlude the conduit in a sequential manner thus creating the peristaltic effect.
Such a mechanism as that shown in FIG. 1 can occupy a relatively large space and includes mechanical inefficiencies caused by the linkages between the drive motor and the work done by the peristaltic fingers. Some of these inefficiencies result from the typical difficulties associated with mechanical drives, such as backlash, slippage, compliance of the belt and tolerances existing in any gear drives, belt drives, and in any mechanical linkages. Additionally, such a belt or gear driven mechanism includes a larger number of parts which consequently increases the size, complexity, cost and weight of the pumping mechanism.
Another prior drive mechanism in fluid infusion systems is the rotating lead screw most often used with syringe pumps. In this arrangement, the output shaft of an electric motor is coupled to an elongated lead screw through a drive belt or gears. A nut device, such as a half-nut, is mounted on the threads of the lead screw and slides along one or more rails which confine the nut device to linear motion. As the lead screw turns, the nut device moves linearly. A pushing arm is coupled to the nut device and engages the plunger of a syringe to press the plunger into the syringe thereby forcing the syringe contents into an administration set for ultimate delivery to a patient.
Such lead screw syringe pumps also suffer from mechanical inefficiencies associated with drive gears, drive belts such as backlash, slippage, compliance of the belt and tolerances existing in any gear drives, belt drives, and in any mechanical linkages. Additionally, such a belt or gear driven mechanism between the motor and the lead screw includes a larger number of parts which consequently increases the size, complexity, cost and weight of the pumping mechanism.
A further consideration is length of battery life. Many pumps are portable and use battery power for operation. Additionally, many other pumps have a battery backup feature which comes into use in the event that the main power for the pump ceases. In both cases, the length of the battery life is an important consideration. In cases where the pump mechanism requires relatively large amounts of power due to mechanical inefficiencies, larger and heavier batteries, or an increased number of batteries are required to obtain the battery life desired. This results in increased weight, size, and expense of the pump. Additionally, recharge times may be extended thus impacting pump service. Such batteries also tend to limit the portability of a pump. Heavier pumps are not considered as portable as lighter pumps. Therefore, mechanical efficiency can greatly affect the performance of a pump from a power consideration.
Hence those concerned with drive mechanisms in fluid infusion have recognized that it would be beneficial to provide a lighter, more efficient and smaller, and less costly pumping mechanism, yet one which is accurate and relatively simple to manufacture. In addition, a more mechanically efficient pump having lessened power requirements is desirable. The present invention fulfills these needs and others.